fixed a bug in dilation and erosion and added soft filters

Since the last release, I fixed a bug in dilation and erosion filtering that was shifting the brightness values downwards by 1. This meant that a binary image (ie. an image with voxel brightnesses of either 0 or 1) that was processed with either dilation or erosion filters would end up as a new image with voxel brightnesses of either -1 or 0. After this fix, the new voxel brightnesses should be between 0 and 1 again.

I also added "soft" versions of the dilation and erosion filters. You can access them from within filter_mrc using the -dilation-binary-soft and -erosion-binary-soft arguments. These only really work well on binary images (whose voxel brightnesses are either 0 or 1).